Self light emitting device and method of driving thereof

ABSTRACT

A self light emitting device having a function of correcting drops in brightness in self light emitting elements in a pixel portion, and capable of displaying a uniform image without brightness irregularities, is provided. A specific test pattern is displayed when an electric power source is connected, brightnesses are detected by photoelectric conversion elements arranged in each pixel, and then stored in a memory circuit. A correction circuit then corrects a first image signal based on portions which are insufficient from standard brightnesses (brightnesses of normal self light emitting elements at the same gray stale, stored in advance), and a second image signal is obtained. Display of an image in a display device is performed in accordance with the second image signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a self light emitting device, and moreparticularly, to an active matrix self light emitting device. Among suchdevices, in particular, the present invention relates to an activematrix self light emitting device using self light emitting elements,such as organic electroluminescence (EL) elements, in a pixel portion.

2. Description of the Related Art

The spread of self light emitting devices in which a semiconductor thinfilm is formed on an insulator such as a glass substrate, in particularto active matrix self light emitting devices using thin film transistors(hereafter referred to as TFTs), has been remarkable recently. Activematrix self light emitting devices have from several hundred thousand toseveral million TFTs formed in a matrix shape in a pixel portion, anddisplay of an image is performed by controlling the electric charge ofeach of the pixels.

In addition, techniques relating to polysilicon TFTs used forsimultaneously forming driver circuits using TFTs formed in theperiphery of the pixel portion, in addition to pixel TFTs structuringthe pixels, have been developed recently, and these contribute greatlyto the miniaturization of devices, and also to reducing the electricpower consumption of the devices. Self light emitting devices have thusbecome indispensable devices in display portions of mobile deviceshaving a remarkably wide range of applications in recent years.

Further, self light emitting devices which apply self light emittingmaterials such as organic EL materials as flat displays in substitutefor LCDs (liquid crystal displays) are under the spotlight, and arebeing enthusiastically researched.

A schematic diagram of a normal self light emitting device is shown inFIG. 15A. The use of an organic EL element (hereafter referred to simplyas an EL element) as one example of a self light emitting element isexplained in this specification. A pixel portion 1504 is arranged in thecenter of an insulating substrate (such as glass, for example) 1501. Inaddition to source signal lines and gate signal lines, electric currentsupply lines 1505 for supplying electric current to EL elements arearranged in the pixel portion 1504. A source signal line driver circuit1502 for controlling the source signal lines is arranged on the top sideof the pixel portion 1504, and gate signal line driver circuits 1503 areplaced on the left and the right of the pixel portion 1504 in order tocontrol the gate signal lines. Note that although the gate signal linedriver circuits 1503 are arranged on both the left and right sides ofthe pixel portion in FIG. 15A, they may also both be placed on the sameside. However, from the perspectives of drive efficiency andreliability, it is preferable to arrange the gate signal lines on bothsides. Input of signals from the outside into the source signal linedriver circuit 1502 and the gate signal line driver circuits 1503 isperformed via a flexible printed circuit (FPC) 1506.

An expanded view of a portion surrounded by a dotted line frame 1500within FIG. 15A is shown in FIG. 15B. The pixel portion has pixelsarranged in a matrix shape, as shown in the figure. A portionadditionally surrounded by a dotted line frame 1510 within FIG. 15B isone pixel, and the pixel has a source signal line 1511, a gate signalline 1512, an electric current supply line 1513, a switching TFT 1514,an EL driver TFT 1515, a storage capacitor 1516, and an EL element 1517.

Operation of active matrix self light emitting devices is explained nextwhile referring to the same FIG. 15B. First, a voltage is applied to thegate electrode of the switching TFT 1514 when the gate signal line 1512is selected, and the switching TFT 1514 is placed in a conductive state.The signal (voltage signal) of the source signal line 1511 is stored asan electric charge in the storage capacitor 1516 by doing so. A voltageV_(GS) between a gate and a source of the EL driver TFT 1515 isdetermined by the electric charge accumulated in the storage capacitor1516, and an electric current corresponding to the voltage of thestorage capacitor 1516 flows in the EL driver TFT 1515 and in the ELelement 1517. The EL element 1517 turns on as a result.

The brightness of the EL element 1517, equal to the amount of electriccurrent flowing in the EL element 1517, can be controlled in accordancewith V_(GS) of the EL driver TFT 1515. V_(GS) is the voltage of thestorage capacitor 1516, and that is the signal (voltage) input to thesource signal line 1511. In other words, the brightness of the ELelement 1517 is controlled by controlling the signal (voltage) input tothe source signal line 1511. Finally, the gate signal line 1512 isplaced in an unselected state, the gate of the switching TFT 1514 isclosed, and the switching TFT 1514 is placed in an unselected state. Theelectric charge which has accumulated in the storage capacitor 1516 ismaintained at this point. V_(GS) of the EL driver TFT 1515 is thereforemaintained as is, and the amount of electric current corresponding toV_(GS) continues to flow in the EL element 1517 via the EL driver TFT1515.

Information regarding EL element drive is reported upon in papers suchas the following: Current Status and Future of Light Emitting PolymerDisplay Driven by Poly-Si TFT, SID99 Digest, p. 372; High ResolutionLight Emitting Polymer Display Driven by Low Temperature PolysiliconThin Film Transistor with Integrated Driver, ASIA DISPLAY 98, p. 217;and 3.8 Green OLED with Low Temperature Poly-Si TFT, Euro Display 99Late News, p. 27.

A method of gray scale display in the EL element 1517 is discussed next.An analog gray scale method for controlling the brightness of the ELelements 1517 by the voltage V_(GS) between the gate and the source ofthe EL driver TFT 1515 has a disadvantage in that it is weak withrespect to dispersion in the electric current characteristics of the ELdriver TFTs 1515. That is, if the electric current characteristics ofthe EL driver TFTs 1515 differ, then the value of the electric currentflowing in the EL driver TFTs 1515 and the EL elements 1517 changes evenif the same gate voltages are applied. As a result, the brightnesses ofthe EL elements 1517, namely the gray scales, also change.

A method referred to as a digital gray scale method has therefore beenproposed in order to reduce the influence of dispersion in thecharacteristics of the EL driver TFTs 1515 and obtain a uniform screenpicture. This method is a method for controlling the gray scale by twostates, a state in which the absolute value |V_(GS)| between a gate anda source of the EL driver TFT 1515 is below the turn on start voltage(in which almost no electric current flows), and a state in which theabsolute value |V_(GS)| is greater than the brightness saturationvoltage (in which an electric current close to the maximum flows). Inthis case, the value of the electric current becomes close to I_(MAX)even if there are dispersions in the electric current characteristics ofthe EL driver TFTs 1515, provided that the absolute values |V_(GS)| ofthe EL driver TFFs 1515 are sufficiently larger than the brightnesssaturation voltage. The influence of EL driver TFT dispersions cantherefore be made extremely small. The gray scales are thus controlledby two states, an ON state (bright state due to maximum electric currentflow) and an OFF state (dark state due to no electric current flow).This method is therefore referred to as a digital gray scale method.

However, only two gray scales can be displayed with the digital grayscale method. A plurality of techniques which can achieve multiple grayscales, in which another method is combined with the digital gray scalemethod, have been proposed.

A time gray scale method is one method which can be used to achievemultiple gray scales. The time gray scale method is a method in whichthe time during which the EL elements 1517 are turned on is controlled,and gray scales are output by the length of the turn on time. In otherwords, one frame period is divided into a plurality of subframe periods,and gray scales are realized by controlling the number and the length ofthe subframe periods during which turn on is performed.

Refer to FIGS. 9A and 9B. Simple timing charts for a time gray scalemethod are shown in FIGS. 9A and 9B. An example of obtaining 3-bit grayscales by a time gray scale method with the frame frequency set to 60 Hzis shown.

As shown in FIG. 9A, one frame period is divided into a number ofsubframe periods corresponding to the number of gray scale bits. Threebits are used here, and therefore one frame period is divided into threesubframe periods SF₁ to SF₃. One subframe period is further divided intoan address period (Ta_(#)) and a sustain (turn on) period (Ts_(#)). (SeeFIG. 20B.) A sustain period during a subframe period denoted byreference symbol SF₁ is referred to as Ts₁. Similarly, sustain periodsfor the cases of subframes SF₂ and SF₃ are referred to as Ts₂ and Ts₃,respectively. Address periods Ta₁ to Ta₃ are each periods during whichone frame portion of an image signal is written into the pixels, andtheir lengths are therefore equal in all of the subframe periods. Thesustain periods have lengths proportional to powers of 2, and thesustain periods here are such that Ts₁:Ts₂:Ts₃=2²:2¹:2⁰=4:2:1.

As a gray scale display method, the brightness is controlled by the sumof all the sustain (turn on) periods within one frame period inaccordance with controlling which subframe periods the EL elements areturned on, and which subframe periods the EL elements are not turned on,in the sustain (turn on) periods from Ts₁ to Ts₃. In this example, 2³=8turn on time lengths can be set by combining the sustain (turn on)periods, and therefore 8 gray scales from 0 (all black display) to 7(all white display) can be displayed, as shown in FIG. 9B. Gray scalesare thus expressed by utilizing the length of the turn on time. Similargray scale expression is also possible, of course, in a color displayself light emitting device.

In addition, the number of divisions within one frame period may also beincreased for a case of increased gray scales. The proportional lengthsof the sustain (turn on) periods for a case of dividing one frame periodinto n subframe periods become Ts₁:Ts₂: . . .:Ts_((n-1)):Ts_(n)=2^((n-1)):2^((n-2)): . . . :2¹:2⁰, and it becomespossible to express 2^(n) gray scales. Note that the appearance of thesubframe periods may be in random order from SF₁ to SFn. Note also thatgray scale expression is possible even if the lengths of the sustain(turn on) periods are not made into powers of two.

Problem points relating to self light emitting devices using self lightemitting elements such as EL elements are discussed. As stated above,electric current is always supplied during the periods in which the ELelements are turned on, and the electric current flows within the ELelements. The nature of the EL elements degrades due to being turned onfor a long time, and the brightness characteristics change with this asa cause. That is, even if the same electric current at the same voltageis supplied from the same electric current supply source, a differencedevelops between the brightness of EL elements which have degraded andEL elements which have not degraded.

A specific example is explained. FIG. 10A is a display screen of adevice using a self light emitting device, such as a portableinformation terminal and icons for operation and the like 1001 aredisplayed. The proportion of time during which there is static displaylike that shown in FIG. 10A is normally large with this type of portableinformation device. If icons and the like are displayed by a color (grayscale) which is brighter than the background, then the EL elements inthe pixels of portions which display the icons and the like are turnedon for a longer time that the EL elements of portions displaying thebackground, and degradation proceeds very quickly.

The EL elements are assumed to have been degraded by these conditions.Display examples of the self light emitting device after degradation areshown in FIGS. 10B and 10C. First, for a case of black display such asthat shown in FIG. 10B, the self light emitting elements such as ELelements are in a state in which a voltage is not applied, namely the ELelements express black by not turning on, and therefore the degradationis not a problem for black display. However, for a case of whitedisplay, even if the same electric current is supplied to the ELelements which have been degraded due to long turn on time (EL elementsof portions displaying icons and the like in this case), there isinsufficient brightness and irregularities develop as shown by referencenumeral 1011 in FIG. 10C.

There is a method in which the voltage applied to the degraded ELelements is raised in order to eliminate the brightness irregularities,but the electric current supply line in a self light emitting device isnormally structured by a single wiring. Further, it is not easy tostructure a circuit for changing the voltage applied to the EL elementin one specific pixel within the pixels arranged in a matrix shape of apixel portion. In addition, there is dispersion of the EL driver TFTs,as stated above, and this correction method cannot be said to bepreferable.

There is a technique recorded in Japanese Patent Application Serial No.2000-273139 as a method of solving the above problem points. A simpleexplanation of this technique is made using FIG. 18.

FIG. 18 is a schematic diagram of a device in a self light emittingdevice having a degradation correction function recorded in JapanesePatent Application Serial No. 2000-273139. In accordance with thismethod, the turn on time of each pixel, or the turn on time and the turnon strength, is detected by periodically sampling a first image signal1801A using a counter 1802 and stored in memories 1803 and 1804. The sumof the detected values, and the hourly change data of the brightnesscharacteristics of the EL elements already stored in a correction datastorage portion 1806 are referenced, the image signal for driving thepixels having degraded EL elements is corrected by computations in acorrection circuit 1805, and a second image signal 1801B is obtained.Image display is performed using the second image signal 1801B. Thebrightness irregularities in the display device 1807 having degraded ELelements in a portion of the pixels are thus corrected, and a uniformscreen picture is obtained.

However, the degradation state of the EL elements at a certain point isnot directly detected with the method stated above, and the degradationstate is merely estimated from the total turn on time of the elements,or from the total turn on time and turn on strength. The term turn onstrength used here is not the turn on strength of the EL elementsthemselves, but is obtained from reading the gray scale of the inputdigital image signal. There is a disadvantage in that the correction ofthe image signal is performed in accordance with correction dataprepared in advance; in other words, degradation not due to driving timecannot be dealt with. For example, reductions in brightness developingfrom degradation due to temperature changes or the like cannot be copedwith by the count of only the total turn on time. There also cannot be aresponse to brightness defects due to dispersion in the initialproperties of the elements themselves with this method.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a self light emittingdevice capable of long term uniform screen display with no brightnessirregularities, in which a degradation state is detected by a methodwhich does not depend on EL element degradation, and used to correct animage signal.

Means such as those below are used in the present invention in order tosolve the above problems.

Pixels have EL elements and photoelectric conversion elements in a selflight emitting device of the present invention having a brightnesscorrection function, and the brightnesses of the EL elements duringdisplay at a certain gray scale are detected by photoelectric conversionelements placed in each pixel. Subsequently, the insufficient brightnessportions are computed by comparing the values detected by thephotoelectric conversion elements with standard brightnesses for the ELelements at the same gray scale which are stored in advance. Aftercorrection is performed on the gray scale data of the image signal by acorrection circuit, it is input to a display device. The display deviceperforms display of an image using the image signal after thecorrection. Uniform display, in which brightness irregularities do notdevelop, can be maintained by the above method even in a self lightemitting device having EL elements which develop brightness defects.

Structures of self light emitting devices of the present invention arerecorded below.

According to a first aspect of the present invention, a self lightemitting device for displaying an image, into which an image signal isinput, is characterized by comprising:

means for detecting the brightnesses of self light emitting elements ofeach pixel;

means for storing the brightnesses; and

means for correcting the image signal based on the stored brightnesses;

wherein:

an image is displayed using the corrected image signal.

According to a second aspect of the present invention, a self lightemitting device for displaying an image, into which an image signal isinput, is characterized by comprising:

a brightness correction device having:

photoelectric conversion elements for detecting the brightnesses of selflight emitting elements of pixels;

a memory circuit for storing the brightnesses of the self light emittingelements of each of the pixels, detected by the photoelectric conversionelements; and

a signal correction portion for correcting a first image signalaccording to the brightnesses of the self light emitting elements ofeach pixel stored in the memory, and outputting a second image signal;and

a display device for performing display of an image based on the secondimage signal.

According to a third aspect of the present invention, a self lightemitting device for displaying an image, into which an image signal isinput, is characterized by comprising:

a brightness correction device having:

j×k (where j and k are natural numbers) photoelectric conversionelements for detecting the brightnesses of self light emitting elementsof each pixel;

a memory circuit for storing the brightnesses of the self light emittingelements of each of the pixels, detected by the photoelectric conversionelements; and

a signal correction portion for correcting a first image signalaccording to the brightnesses of the self light emitting elements ofeach pixel stored in the memory, and outputting a second image signal;and

a display device having j×k pixels for performing display of an imagebased on the second image signal.

According to a fourth first aspect of the present invention, a selflight emitting device according to any one of claims 1 to 3 ischaracterized in that:

the self light emitting device performs display of n bit (where n is anatural number, n≧2) gray scales and has a driver circuit for performingn+m bit (where m is a natural number) signal processing; and

equal brightnesses are obtained between self light emitting elementswhich do not develop a reduction in brightness, and self light emittingelements which do develop a reduction in brightness, by:

performing display of gray scales in accordance with an n bit imagesignal in the pixels having light emitting elements which do not developa reduction in brightness, and

performing correction of the image signal by using an m bit signalagainst the n bit image signal in the pixels in which do develop areduction in brightness.

According to a fifth aspect of the present invention, in the self lightemitting device according to any one of the first to third aspects ofthe invention, the device is characterized in that the correction meansperforms relative addition processing on image signals written into thepixels of the self light emitting elements which develop a reduction inbrightness, with respect to image signals written into the pixels of theself light emitting elements which do not develop a reduction inbrightness.

According to a sixth aspect of the present invention, in the self lightemitting device according to any one of the first to third aspects ofthe invention, the device is characterized in that the correction meansperforms relative subtraction processing on image signals written intothe pixels of the self light emitting elements which develop a smallreduction in brightness, or on image signals written into the pixels ofthe self light emitting elements which do not develop a reduction inbrightness, with respect to image signals written into the pixels of theself light emitting elements which develop a large reduction inbrightness.

According to a seventh aspect of the present invention, in the selflight emitting device according to any of the first to sixth aspects ofthe invention, the device is characterized in that the storage meansuses a static memory circuit (SRAM).

According to an eighth aspect of the present invention, in the selflight emitting device according to any of the first to sixth aspect ofthe invention, the device is characterized in that the storage meansuses a dynamic memory circuit (DRAM).

According to a ninth aspect of the present invention, in the self lightemitting device according to any of the first to sixth aspect of theinvention, the device is characterized in that the storage means uses aferroelectric memory circuit (FeRAM).

According to a tenth aspect of the present invention, in the self lightemitting device according to any of the first to sixth aspects of theinvention, the device is characterized in that the storage means uses anon-volatile memory (EEPROM) capable of being electrically written into,read out form, and erased.

According to an eleventh aspect of the present invention, tin he selflight emitting device according to any one of the first to tenth aspectsof the invention, the device is characterized in that PN photodiodes areused in the photoelectric conversion elements as the brightnessdetection means.

According to a twelfth aspect of the present invention, in the selflight emitting device according to any one of the first to tenth aspectsof the invention, the device is characterized in that PIN photodiodesare used in the photoelectric conversion elements as the brightnessdetection means.

According to a thirteenth aspect of the present invention, in the selflight emitting device according to any one of the first to tenth aspectsof the invention, the device is characterized in that avalanchephotodiodes are used in the photoelectric conversion elements as thebrightness detection means.

According to a fourteenth aspect of the present invention, in the selflight emitting device according to any one of the first to thirteenthaspects of the invention, the device is characterized in that thedetection means, the storage means, and the correction means arestructured by circuits external to the self light emitting device.

According to a fifteenth aspect of the present invention, in the selflight emitting device according to any one of the first to thirteenthaspects of the invention, the device is characterized in that thedetection means, the storage means, and the correction means are formedon a same insulator as the self light emitting device.

According to a sixteenth aspect of the present invention, in the selflight emitting device according to any one of the first to fifteenthaspects of the invention, the device is characterized in that the selflight emitting device is an EL display.

According to a seventeenth aspect of the present invention, in the selflight emitting device according to any one of the first to fifteenthaspects of the invention, the device is characterized in that the selflight emitting device is a PDP display.

According to an eighteenth aspect of the present invention, in the selflight emitting device according to any one of the first to fifteenthaspects of the invention, the device is characterized in that the selflight emitting device is an FED display.

According to a nineteenth aspect of the present invention, a method ofdriving a self light emitting device for displaying an image, into whichan image signal is input, is characterized by:

detecting the brightnesses of self light emitting elements of eachpixel;

storing the detected brightnesses of the self light emitting elements ofeach of the pixels;

performing correction according to the difference between the storedbrightnesses of the self light emitting elements of each of the pixels,and standard brightnesses, and outputting a second image signal; and

performing display of an image using the second image signal.

According to a twentieth aspect of the present invention, a method ofdriving a self light emitting device for displaying an image, into whichan image signal is input, is characterized by:

detecting the brightnesses of self light emitting elements of each pixelby using photoelectric conversion elements;

storing the brightnesses of the self light emitting elements of each ofthe pixels detected by the photoelectric conversion elements, in amemory circuit;

performing correction of a first image signal in a signal correctionportion according to the difference between the brightnesses of the selflight emitting elements of each of the pixels stored in the memorycircuit and standard brightnesses, and outputting a second image signal;and

performing display of an image using the second image signal.

According to a twenty-first aspect of the present invention, in themethod of driving a self light emitting device according to thenineteenth or twentieth aspect of the invention, the method ischaracterized in that:

the self light emitting device performs display of n bit (where n is anatural number, n≧2) gray scales and has a driver circuit for performingn+m bit (where m is a natural number) signal processing; and

equal brightnesses are obtained between self light emitting elementswhich do not develop a reduction in brightness, and self light emittingelements which do develop a reduction in brightness, by:

performing display of gray scales in accordance with an n bit imagesignal in the pixels having self light emitting elements which do notdevelop a reduction in brightness, and

performing correction of the image signal by using an m bit signalagainst the n bit image signal in the pixels having self light emittingelements which do develop a reduction in brightness.

According to a twenty-second aspect of the present invention, in themethod of driving a self light emitting device according to any one ofthe nineteenth to twenty-first aspects of the invention, the method ischaracterized in that the correction means performs relative additionprocessing on image signals written into the pixels of the self lightemitting elements which develop a reduction in brightness, with respectto image signals written into the pixels of the self light emittingelements which do not develop a reduction in brightness.

According to a twenty-third aspect of the present invention, in themethod of driving a self light emitting device according to any one ofthe nineteenth to twenty-first aspects of the invention, the method ischaracterized in that the correction means performs relative subtractionprocessing on image signals written into the pixels of the self lightemitting elements which develop a small reduction in brightness, or onimage signals written into the pixels of the self light emittingelements which do not develop a reduction in brightness, with respect toimage signals written into the pixels of the self light emittingelements which develop a large reduction in brightness.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram of a self light emitting device of the presentinvention having brightness detection and correction functions;

FIGS. 2A to 2E are diagrams showing a correction method in accordancewith addition processing;

FIGS. 3A to 3E are diagrams showing a correction method in accordancewith subtraction processing;

FIGS. 4A and 4B are a block diagram of a display device in a self lightemitting device of the present invention having brightness detection andcorrection functions, and an equivalent circuit diagram of a pixelportion, respectively;

FIGS. 5A to 5C are diagrams showing an example of a process of producingan active matrix self light emitting device;

FIGS. 6A to 6C are diagrams showing the example of a process ofproducing an active matrix self light emitting device;

FIGS. 7A and 7B are diagrams showing the example of a process ofproducing an active matrix self light emitting device;

FIG. 8 is a diagram showing the example of a process of producing anactive matrix self light emitting device;

FIGS. 9A and 9B are diagrams for explaining a time gray scale method;

FIGS. 10A to 10C are diagrams showing the generation of brightnessirregularities of a screen picture in accordance with degradation oflight emitting elements;

FIGS. 11A to 11F are diagrams showing examples of applications of theself light emitting device of the present invention having brightnessdetection and correction functions to electronic equipment;

FIGS. 12A to 12C are diagrams showing examples of applications of theself light emitting device of the present invention having brightnessdetection and correction functions to electronic equipment;

FIG. 13 is a block diagram of a self light emitting device of thepresent invention having brightness detection and correction functions;

FIGS. 14A and 14B are block diagrams of source signal line drivercircuits of a digital image signal input method and an analog signalinput method, respectively, in a self light emitting device of thepresent invention having brightness detection and correction functions;

FIGS. 15A and 15B are diagrams showing an example of conventional selflight emitting devices;

FIG. 16 is a diagram showing an example of a wiring pattern of a pixelportion in a self light emitting device of the present invention havingbrightness detection and correction functions;

FIG. 17 is a diagram showing an example of a production process of anactive matrix self light emitting device; and

FIG. 18 is a block diagram of a self light emitting device having acorrection function and recorded in Japanese Patent Application SerialNo. 2000-273139.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment Mode

Refer to FIG. 1. FIG. 1 shows a block diagram of a self light emittingdevice of the present invention having a brightness correction function.A brightness correction device, the mainstay of the present invention,is composed of a memory circuit portion 100, a correction circuit 105,and photoelectric conversion elements 106. The memory circuit portion100 houses a correction data storage portion 102 a test pattern 103 andthe like and has a memory circuit 104 for storing detected brightnesses.The photoelectric conversion elements 106 are arranged so that theyoverlap with a portion of the self light emitting elements 107. Thelight emitting surface of the self light emitting elements 107 iscompressed if the size of the photoelectric conversion elements 106 islarge, and it is therefore preferable to make the photoelectricconversion elements 106 as small as possible. An amplified voltage isobtained through an amplifier circuit, such as an operational amplifier,for signals resulting after photoelectric conversion of light irradiatedfrom the self light emitting elements 107 because the signals are weak.

A circuit diagram of a source signal line driver circuit in a displaydevice 108 is shown in FIG. 14A. Here, a display device corresponding toa digital image signal is exemplified. The source signal line drivercircuit has a shift register (SR) 1401, a first latch circuit (LAT1)1402, and a second latch circuit (LAT2) 1403. Reference numeral 1404denotes a pixel, and reference numeral 1405 is a brightness correctiondevice shown in FIG. 1.

Operations of each portion are explained. Sampling pulses are output oneafter another from the shift register in accordance with a clock signal(CLK) and a start pulse (SP). Storage of a digital image signal isperformed in the first latch circuit in accordance with the timing ofthe sampling pulses. Correction of the image signal is already completedat this point, and it becomes a second image signal, as shown in FIG.14A. A latch pulse is output when a storage of one horizontal periodportion is complete in the first latch circuit, and the digital imagesignal is transferred to the second latch circuit. Thereafter, write infrom the second latch circuit to the pixels is performed. At the sametime, storage of the digital image signal in the first latch circuit isagain performed in accordance with the sampling pulse from the shiftregister.

Operation of the overall brightness correction device is explained next.First, the brightness with respect to the input of a certain gray scalesignal is stored in the correction data storage portion 102 in advance,as a standard brightness for the EL elements used in the self lightemitting device. Correction of the image signal is performed for the ELelements of each pixel in accordance with the deviation from thestandard brightnesses. Further, the standard brightnesses need not belimited to one certain gray scale, but standard brightnesses may bestored for a plurality of gray scales.

A test pattern is next input to the display device, and screen displayis performed. It is preferable that the test pattern be a pattern suchas a plain intermediate display or a white display. The above-mentionedstandard brightnesses are the standard brightnesses occurring in thosegray scales. In addition to the standard brightnesses, the amount ofchange in the brightness per single gray scale in a certain number ofbits is also stored in the correction data storage portion 102. Resultsof detection here are temporarily stored in the memory circuit 104. Thebrightnesses are then detected by the photoelectric conversion elementsformed in each pixel in the pixel portion while the EL elements areturned on in accordance with the test pattern. For example, ifdegradation develops in a certain EL element due to any cause, normallythe brightness of the EL element will drop. A difference in brightnesstherefore arises between the detected brightness and the standardbrightness even if the same gray scale signal is displayed. Thebrightness difference is computed for several gray scales of the digitalimage signal currently used, correction is added to a first image signal101A for the computed gray scales in each pixel, and a second imagesignal 101B is obtained, and then input to the display device.

It is necessary to use non-volatile memory such as flash memory in thecorrection data storage portion 102 and for the test pattern 103 instructuring the memory circuit portion 100. Further, the brightnessdetection results are always refreshed when the electric power source isconnected as stated above, and therefore volatile memory may also beused in the memory circuit 104. Memory such as static memory (SRAM),dynamic memory (DRAM), and ferroelectric memory (FRAM) may be used asvolatile memory. However, there are no specific limitations placed onthe structure of the memory circuit in the present invention.

A brightness detection procedure by the photoelectric conversionelements 106 is as follows. It is preferable that refresh of the memorycircuit 104 is always performed for times of normal image display. It ispreferable to perform correction of the image signal in real time, butconsidering the actual operation of the photoelectric conversionelements 106, this is difficult due to the amount of time required, andtherefore an example of performing the above procedures when theelectric power source is connected to the self light emitting device isgiven as one method. It is of course possible to know to what extent thebrightnesses of the EL elements have dropped by comparing the firstimage signal with brightnesses detected in real time during display ofthe image obtained by input of the first image signal, provided that thephotoelectric conversion elements used have a fast response time.Correction operations can therefore also be performed during imagedisplay.

Note that characteristics such as small size, high speed response,stability, linearity with respect to incident light, and high detectionsensitivity are sought for the photoelectric conversion elements used inthe self light emitting device of the present invention. It is thereforepreferable to use photodiodes in the self light emitting device of thepresent invention due to these requirements. In particular, PN junctionphotodiodes and PIN junction photodiodes are easily formed duringprocessing, as explained later in embodiments, and can be formed verysmall, and as such, are particularly preferable. Note that diodes suchas avalanche diodes can also be given as other photodiodes, and that anyof these photodiodes may be used in structuring the present invention.

Furthermore, a switch 113 is used in switching the inputs between thetest pattern and the normal digital image signal in the figures shown bythe embodiment mode, but there are no particular limitations placed onthis, and other methods may also be used.

Embodiments

Embodiments of the present invention are discussed below.

Embodiment 1

A method of achieving a brightness equal to that of a normal EL elementby adding a certain corrective value to an input digital image signaland effectively converting it to a signal of several gray scales higher,can be given as one method of correcting insufficient brightness indegraded EL elements by using the image signal level. The easiest methodto realize this by circuit design, a circuit capable of processing thesuperimposition gray scales is prepared beforehand. Specifically, for acase of a six bit digital gray scale (64 gray scales) self lightemitting device of the present invention having a brightness correctionfunction, for example, a one bit of processing capability is added assuperimposition means for performing correction. This is effectivelydesigned as seven bit digital gray scales (128 gray scales),manufactured, and in normal operation, the lower six bits are used. Ifan EL element develops degradation, the corrective value is added to thenormal digital image signal, and signal processing of the added portionis performed using the above one bit for superimposition. The mostsignificant bit (MSB) is used only for signal correction in this case,and the actual display gray scale is six bit.

Embodiment 2

A method of correcting a digital image signal, differing from the methodof embodiment 1, is explained in embodiment 2.

Refer to FIG. 1 and FIGS. 2A to 2E. FIG. 2A shows a portion of a pixelof the display device 108 in FIG. 1. Note that, for simplicity,photoelectric conversion elements arranged in a pixel portion are notshown in the figures.

Three pixels, pixels 201 to 203, are considered here. First, the pixel201 is a pixel which does not develop degradation, while both the pixels202 and 203 develop a certain amount of degradation. If the amount ofdegradation in the pixel 203 is taken as being larger than that of thepixel 202, then the brightness reduction also naturally becomes largeralong with the degradation. In other words, if a certain intermediategray scale is displayed, then brightness irregularities develop as inFIG. 2B. The brightness of the pixel 202 becomes lower than thebrightness of the pixel 201, and further, the brightness of the pixel203 becomes lower.

Actual correction operation is explained next. Brightness correction byaddition processing is explained first.

Values of the brightness of EL elements which are turned on due to acertain gray scale signal are measured in advance, and taken as standardbrightnesses. The standard brightnesses are stored in the correctiondata storage portion 102 along with the amount of change in thebrightness per one gray scale of a certain digital image signal. Displayis then performed in accordance with a certain test pattern, and thebrightnesses of each pixel within the screen are detected by thephotoelectric conversion elements 106, and converted into signals. Thestandard brightnesses and the results of detecting the brightnesses ineach pixel are input to the correction circuit 105. The results ofdetecting the brightness in each pixel are temporarily stored in thememory circuit 104, and then read out and input to the correctioncircuit 105.

Computation is performed next in the correction circuit 105 from theinput values, the amount of correction to the digital image signalwritten into each pixel is determined, and actual correction isperformed. One example is shown in FIG. 2C. With respect to a standardbrightness A, the brightness of the pixel 201 is taken as B₁, thebrightness of the pixel 202 is taken as B₂, and the brightness of thepixel 203 is taken as B₃. The correction widths for the digital imagesignal are found by taking the differences between the standardbrightness A and the detected brightnesses B₁ to B₃, and dividing byamounts of brightness change X per unit gray scale. Here, the correctionamount is 0 for the pixel 201, the correction amount is 1 for the pixel202, and the correction amount is 2 for the pixel 203, as shown in FIG.2C. The brightness differences are approximated when falling within onegray scale to determine the amounts of correction. In this case, thebrightness of a 0.5 gray scale portion is taken as a boundary, forexample, and rounding up or rounding down may be selected or uniformprocessing may be performed by selecting one of them.

Brightness correction is performed by determining the amount ofcorrection widths in each pixel by the above method, and successivelyadding correction signals to the gray scale signals with the first imagesignal 101A input to the correction circuit 105. As shown in FIGS. 2Dand 2E, the digital image signals input to each pixel are superimposedby gray scales for the correction widths found, and brightnesses equalto those of normal EL elements are obtained. The second image signal101B, with which correction is thus completed, is then input to thedisplay device 108 and image display is performed.

A correction method in accordance with subtraction processing isdiscussed next. FIG. 1 and FIGS. 3A to 3E are referred to. FIGS. 3A and3B are identical to FIGS. 2A and 2B, respectively, and therefore anexplanation of FIGS. 3A and 3B is omitted here.

Similar to addition processing discussed above, the brightnesses of eachpixel are detected by the photoelectric conversion elements, are readinto the correction circuit along with the standard brightnesses, andcorrection of the digital image signal is performed. The brightness inthe pixel thought to have the most advanced degradation (the lowestbrightness) in the pixel portion is taken as the standard brightness atthis point. The brightness of a pixel 301 is taken as B₁, the brightnessof a pixel 302 is taken as B₂, and the brightness of a pixel 303 istaken as B₃ with respect to a standard brightness C. The correctionwidths for the digital image signal are found by taking the differencesbetween the standard brightness C and the detected brightnesses B₁, toB₃, and dividing by amounts of brightness change X per unit gray scale.The correction amount is −2 for the pixel 301 here, the correctionamount is −1 for the pixel 302, and the correction amount is 0 for thepixel 303, as shown in FIG. 3C. The brightness differences areapproximated when falling within one gray scale to determine the amountsof correction. In this case, the brightness of a 0.5 gray scale portionis taken as a boundary, for example, and rounding up or rounding downmay be selected or uniform processing may be performed by selecting oneof them.

Brightness correction is performed by determining the correction widthsin each pixel by the above method, and successively lowering the grayscales of the digital image signal by the amount of correction with thefirst image signal 101A input to the correction circuit 105. As shown inFIGS. 2D and 2E, the gray scales for the correction widths found aresubtracted from the digital image signals input to each pixel, and thebrightnesses are suppressed to brightnesses equal to that of the ELelement in which the brightness has dropped the most. The second imagesignal 101B, with which correction is thus completed, is then input tothe display device 108.

However, if correction is performed in accordance with the aboveprocedure, the brightness of the overall screen becomes reduced byseveral gray scales (the difference between the gray scale of theoriginal digital image signal and the gray scale of the second imagesignal written into the pixels in which EL element degradation has notdeveloped). A normal, uniform screen picture is obtained as shown inFIG. 3E in accordance with correcting the brightness of the entirescreen by slightly increasing the voltage V_(EL) between both electrodesof the EL elements (V_(EL1)+δ→V_(EL2)) by changing the electricpotential of the electric current supply line, as shown in FIG. 3D.

For the former case of correction by addition processing, it is possibleto correct the brightness irregularities by only performing digitalimage signal processing, but there is a disadvantage in that correctionin white display is not effective (specifically, for a case of a six bitdigital image signal, for example, in which “111111” is input, it is notpossible to make further additions). Further, for the latter case ofcorrection by subtraction processing, electric potential control of theelectric current supply line is added for brightness correction, but therange in which correction is not effective is the black display range,the opposite of the addition processing correction, and therefore thereare almost no poor influences. (specifically, for a case of a six bitdigital image signal, for example, in which “000000” is input, it is notnecessary to perform further subtractions, and normal black display ispossible between normal EL elements and EL elements which have degraded(the EL elements are simply in a non-turned on state). Further, therewill be almost no problems for a number of gray scales in the vicinityof black, provided that the number of corresponding bits of the displaydevice is somewhat high.) Both methods are advantageous methods formultiple gray scales.

Further, for example, it is also effective to suppress the demerits ofboth methods by using them jointly in a method of correction with bothaddition processing and subtraction processing, taking a certain grayscale as a boundary.

On the other hand, after displaying a test pattern temporarily when theelectric power source is connected and detecting the brightnesses ofeach pixel, the image signal input system is switched over to normaloperation (performed by the switch 113 shown in FIG. 1 with the examplesof this specification), a digital image signal is input, and imagedisplay is performed.

Embodiment 3

Details of the display device 108 in the schematic diagram shown in FIG.1 are explained using FIGS. 4A and 4B. FIG. 4A is a schematic diagram ofthe entire display device, and FIG. 4B is an equivalent circuit diagramof a pixel portion. A pixel portion 405 is arranged in the center of asubstrate 400 in FIG. 4A. Pixels 406, each having an EL element and aphotoelectric conversion element, are arranged in a matrix shape in thepixel portion 405, as explained later. An EL source signal line drivercircuit 401, an EL gate signal line driver circuit 402, a photoelectricconversion element signal line driver circuit 403, and a photoelectricconversion element scanning line driver circuit 404 are arranged in theperiphery of the pixel portion 405. One each of the driver circuits arearranged in the periphery of the pixel portion in embodiment 3, butdifferent circuit arrangements may also be used, such as the EL sourcesignal line driver circuit 401 and the photoelectric conversion elementsignal line driver circuit 403, or the EL gate signal line drivercircuit 402 and the photoelectric conversion element scanning linedriver circuit 404, for example, being integrated into one circuit, andthe circuits being arranged on both sides opposing the pixel portion.Supply of signals and an electric power source to each of the drivercircuits is performed through an FPC 407.

FIG. 4B is a blow up of the pixels 406. One pixel is structured by asource signal line 411, a gate signal line 412, a switching TFT 413, anEL driver TFT 414, a storage capacitor 415, an EL element 416, anelectric current supply line 417, a signal output line 418, a resetsignal line 419, a scanning line 420, a standard electric power sourceline 421, a reset TFT 422, a buffer TFT 423, a selection TFT 424, and aphotoelectric conversion element 425. The storage capacitor 415 isarranged in order to store an electric charge imparted to a gateelectrode of the EL driver TFT 414, but it is not always necessary forthe storage capacitor 415 to be used.

Turn on of the EL element has already been discussed, and will thereforebe omitted here. Only operation in the periphery of the photoelectricconversion element during brightness detection in each pixel isdiscussed here. The selection TFT 424 is placed in a conductive statewhen a selection pulse is input to the scanning line 420. Light from theEL element 416 is made incident to the photoelectric conversion element425 with this state, the buffer TFT 423 is made conductive in accordancewith the electric charge accumulated in the photoelectric conversionelement 425, and the brightness is converted to an electric signal andoutput to the signal output line 418. The signal is amplified using abuffer, an operational amplifier and the like in the signal line drivercircuit 403, and a voltage signal is obtained. This is then read into acorrection circuit through means such as an A/D converter.

Embodiment 4

With the example shown by the embodiment mode (FIG. 1), the brightnesscorrecting device is placed outside the display device 108 in the selflight emitting device of the present invention having a brightnesscorrection function, and the digital image signal (first image signal)101A is first input to the correction circuit 105, where correction isimmediately performed, and then the corrected digital image signal(second image signal) 101B is input to the display device 108 through anFPC. Examples of merits of this method include a high amount ofcompatibility due to making each device into a unit, and goodapplicability, but on the other hand, lower costs, conservation ofspace, and high speed drive can be realized due to a large reduction inthe number of parts by forming the brightness correction device and thedisplay device integrated on the same substrate. No particular layout onthe substrate is shown here, but considering such things as thearrangement of signal lines and the length of wiring, it is preferableto have adjacent arrangement block by block.

Embodiment 5

In Embodiment 5, a method of manufacturing TFTs of a pixel portion, adriver circuit portion (source signal line driver circuit, gate signalline driver circuit and pixel selection signal line driver circuit)formed in the periphery thereof in an active EL display device of thepresent invention simultaneously is explained. Note that a CMOS circuitwhich is a base unit is illustrated as the driver circuit portion tomake a brief explanation.

First, as shown in FIG. 5A, a substrate 5000 is used, which is made ofglass such as barium borosilicate glass or alumino borosilicate glass,typified by #7059 glass or #1737 glass of Corning Inc. There is nolimitation on the substrate 5000 as long as a substrate having a lighttransmitting property is used, and a quartz substrate may also be used.In addition, a plastic substrate having heat resistance to a treatmenttemperature of this embodiment may also be used.

Then, a base film 5001 formed of an insulating film such as a siliconoxide film, a silicon nitride film or a silicon oxynitride film isformed on the substrate 5000. In this embodiment, a two-layer structureis used for the base film 5001. However, a single layer film or alamination structure consisting of two or more layers of the insulatingfilm may also be used. As a first layer of the base film 5001, a siliconoxynitride film 5001 a is formed with a thickness of 10 to 200 nm(preferably 50 to 100 nm) using SiH₄, NH₃, and N₂O as reaction gases bya plasma CVD method. In this embodiment, the silicon oxynitride film5001 a (composition ratio Si=32%, O=27%, N=24% and H=17%) having a filmthickness of 50 nm is formed. Then, as a second layer of the base film5001, a silicon oxynitride film 5001 b is formed so as to be laminatedon the first layer with a thickness of 50 to 200 nm (preferably 100 to150 nm) using SiH₄ and N₂O as reaction gases by the plasma CVD method.In this embodiment, the silicon oxynitride film 5001 b (compositionratio Si=32%, O=59%, N=7% and H=2%) having a film thickness of 100 nm isformed.

Subsequently, semiconductor layers 5002 to 5004 are formed on the basefilm. The semiconductor layers 5002 to 5004 are formed such that asemiconductor film having an amorphous structure is formed by a knownmethod (a sputtering method, an LPCVD method, a plasma CVD method or thelike), and is subjected to a known crystallization process (a lasercrystallization method, a thermal crystallization method, a thermalcrystallization method using a catalyst such as nickel, or the like) toobtain a crystalline semiconductor film, and the crystallinesemiconductor film is patterned into desired shapes. The semiconductorlayers 5002 to 5004 are formed with a thickness of 25 to 80 nm(preferably 30 to 60 nm). The material of the crystalline semiconductorfilm is not particularly limited, but it is preferable to form the filmusing silicon, a silicon germanium (Si_(x)Ge_(1-x) (X=0.0001 to 0.02))alloy, or the like. In this embodiment, an amorphous silicon film of 55nm thickness is formed by a plasma CVD method, and then, anickel-containing solution is held on the amorphous silicon film. Adehydrogenation process of the amorphous silicon film is performed (at500° C. for 1 hour), and thereafter a thermal crystallization process isperformed (at 550° C. for 4 hours) thereto. Further, to improve thecrystallinity, a laser annealing process is performed to form thecrystalline silicon film. Then, this crystalline silicon film issubjected to a patterning process using a photolithography method toobtain the semiconductor layers 5002 to 5004.

Further, after the formation of the semiconductor layers 5002 to 5004, aminute amount of impurity element (boron or phosphorus) may be doped tocontrol a threshold value of the TFT.

Besides, in the case where the crystalline semiconductor film ismanufactured by the laser crystallization method, a pulse oscillationtype or continuous emission type excimer laser, YAG laser, or YVO₄ lasermay be used. In the case where those lasers are used, it is appropriateto use a method in which laser light radiated from a laser oscillator iscondensed into a linear shape by an optical system, and is irradiated tothe semiconductor film. Although the conditions of crystallizationshould be properly selected by an operator, in the case where theexcimer laser is used, a pulse oscillation frequency is set to 30 Hz,and a laser energy density is set to 100 to 400 mJ/cm² (typically 200 to300 mJ/cm²). In the case where the YAG laser is used, it is appropriateto set a pulse oscillation frequency as 1 to 10 Hz using the secondharmonic, and to set a laser energy density to 300 to 600 mJ/cm²(typically, 350 to 500 mJ/cm²). Then, laser light condensed into alinear shape with a width of 100 to 1000 μm, for example, 400 μm, isirradiated to the whole surface of the substrate, and an overlappingratio (overlap ratio) of the linear laser light at this time may be setto 50 to 90%.

A gate insulating film 5005 is then formed for covering thesemiconductor layers 5002 to 5004. The gate insulating film 5005 isformed of an insulating film containing silicon with a thickness of 40to 150 nm by a plasma CVD or sputtering method. In this embodiment, thegate insulating film 5005 is formed of a silicon oxynitride film with athickness of 110 nm by the plasma CVD method (composition ratio Si=32%,O=59%, N=7%, and H=2%). Of course, the gate insulating film is notlimited to the silicon oxynitride film, and other insulating filmscontaining silicon may be used with a single layer or a laminationstructure.

Besides, when a silicon oxide film is used, it can be formed such thatTEOS (tetraethyl orthosilicate) and O₂ are mixed by the plasma CVDmethod with a reaction pressure of 40 Pa and a substrate temperature of300 to 400° C., and discharged at a high frequency (13.56 MHz) powerdensity of 0.5 to 0.8 W/cm². The silicon oxide film thus manufacturedcan obtain satisfactory characteristics as the gate insulating film bysubsequent thermal annealing at 400 to 500° C.

Then, a first conductive film 5006 of 20 to 100 nm thickness and asecond conductive film 5007 of 100 to 400 nm thickness are formed intolamination on the gate insulating film 5005. In this embodiment, thefirst conductive film 5006 made of a TaN film with a thickness of 30 nmand the second conductive film 5007 made of a W film with a thickness of370 nm are formed into lamination. The TaN film is formed by sputteringwith a Ta target under a nitrogen containing atmosphere. Besides, the Wfilm is formed by sputtering with a W target. The W film may also beformed by a thermal CVD method using tungsten hexafluoride (WF₆).Whichever method is used, it is necessary to make the material have lowresistance for use as a gate electrode, and it is preferred that theresistivity of the W film is set to 20 μΩcm or less. It is possible tomake the W film have low resistance by making the crystal grains large.However, in the case where many impurity elements such as oxygen arecontained within the W film, crystallization is inhibited and theresistance becomes higher. Therefore, in this embodiment, the W film isformed by sputtering using a W target having a high purity of 99.9999%,and also by taking sufficient consideration so as to prevent impuritieswithin the gas phase from mixing therein during the film formation, andthus, a resistivity of 9 to 20 μΩcm can be realized.

Note that, in this embodiment, the first conductive film 3006 is made ofTaN, and the second conductive film 5007 is made of W, but the materialis not particularly limited thereto, and either film may be formed froman element selected from the group consisting of Ta, W, Ti, Mo, Al, Cu,Cr, and Nd or an alloy material or a compound material containing theabove element as its main constituent. Besides, a semiconductor filmtypified by a polycrystalline silicon film doped with an impurityelement such as phosphorus may be used. An alloy made of Ag, Pd, and Cumay also be used. Further, any combination may be employed such as acombination in which the first conductive film is formed of a tantalum(Ta) film and the second conductive film is formed of a W film, acombination in which the first conductive film is formed of a titaniumnitride (TiN) film and the second conductive film is formed of a W film,a combination in which the first conductive film is formed of a tantalumnitride (TaN) film and the second conductive film is formed of an Alfilm, or a combination in which the first conductive film is formed of atantalum nitride (TaN) film and the second conductive film is formed ofa Cu film.

Next, as shown in FIG. 5B, masks 5008 made of resist are formed by usinga photolithography method, and a first etching process for formingelectrodes and wirings is carried out. In the first etching process,first and second etching conditions are used. In this embodiment, as thefirst etching condition, an ICP (inductively coupled plasma) etchingmethod is used, in which CF₄, Cl₂, and O₂ are used as etching gases, agas flow rate is set to 25/25/10 sccm, and an RF (13.56 MHz) power of500 W is applied to a coil shape electrode under a pressure of 1 Pa togenerate plasma. Thus, the etching is performed. A dry etching deviceusing ICP (Model E645-ICP) manufactured by Matsushita ElectricIndustrial Co. is used here. A 150 W RF (13.56 MHz) power is alsoapplied to the substrate side (sample stage), thereby substantiallyapplying a negative self-bias voltage. The W film is etched under thefirst etching condition, and the end portion of the first conductivelayer is formed into a tapered shape. In the first etching condition,the etching rate for W is 200.39 nm/min, the etching rate for TaN is80.32 nm/min, and the selectivity of W to TaN is about 2.5. Further, thetaper angle of W is about 26° under the first etching condition.

Thereafter, as shown in FIG. 5B, the etching condition is changed intothe second etching condition without removing the masks 5008 made ofresist, and the etching is performed for about 30 seconds, in which CF₄and Cl₂ are used as the etching gases, a gas flow rate is set to 30/30sccm, and an RF (13.56 MHz) power of 500 W is applied to a coil shapeelectrode under a pressure of 1 Pa to generate plasma. An RF (13.56 MHz)power of 20 W is also applied to the substrate side (sample stage), anda substantially negative self-bias voltage is applied thereto. In thesecond etching condition in which CF₄ and Cl₂ are mixed, the W film andthe TaN film are etched to the same degree. In the second etchingcondition, the etching rate for W is 58.97 nm/min, and the etching ratefor TaN is 66.43 nm/min. Note that, in order to perform the etchingwithout leaving any residue on the gate insulating film, it isappropriate that an etching time is increased by approximately 10 to20%.

In the above first etching process, by making the shapes of the mask5008 formed of resist suitable, end portions of the first conductivelayer and the second conductive layer become tapered shape by the effectof the bias voltage applied to the substrate side. The angle of thetaper portion may be 15 to 45°. In this way, first shape conductivelayers 5009 to 5013 consisting of the first conductive layer and thesecond conductive layer (first conductive layers 5009 a to 5013 a andsecond conductive layers 5009 b to 5013 b) are formed by the firstetching process. Reference numeral 5005 indicates a gate insulatingfilm, and the regions not covered with the first shape conductive layers5009 to 5013 are made thinner by approximately 20 to 50 nm by etching.

Then, a first doping process is performed to add an impurity elementimparting n-type conductivity to the semiconductor layer withoutremoving the mask 5008 made of resist (FIG. 5B). Doping may be carriedout by an ion doping method or an ion injecting method. The condition ofthe ion doping method is that a dosage is 1×10¹³ to 5×10¹⁵ atoms/cm²,and an acceleration voltage is 60 to 100 keV. In this embodiment, thedosage is 1.5×10¹⁵ atoms/cm² and the acceleration voltage is 80 keV. Asthe impurity element imparting n-type conductivity, an element belongingto group 15 of the periodic table, typically phosphorus (P) or arsenic(As) is used, but phosphorus (P) is used here. In this case, theconductive layers 5009 to 5012 become masks for the impurity elementimparting n-type conductivity, and high concentration impurity regions5014 to 5016 are formed in a self-aligning manner. The impurity elementimparting n-type conductivity in a concentration range of 1×10²⁰ to1×10²¹ atoms/cm³ is added to the high concentration impurity regions5014 to 5016.

Thereafter, as shown in FIG. 5C, a second etching process is performedwithout removing the masks made of resist. Here, a gas mixture of CF₄,Cl₂ and O₂ is used as an etching gas, the gas flow rate is set to20/20/20 sccm, and a 500 W RF (13.56 MHz) power is applied to a coilshape electrode under a pressure of 1 Pa to generate plasma, therebyperforming etching. A 20 W RF (13.56 MHz) power is also applied to thesubstrate side (sample stage), thereby substantially applying a negativeself-bias voltage. In the second etching process, the etching rate for Wis 124 nm/min, the etching rate for TaN is 20 nm/min, and theselectivity of W to TaN is 6.05. Accordingly, the W film is selectivelyetched. The taper angle of W is 70° by the second etching process.Second conductive layers 5017 b to 5021 b are formed by the secondetching process. On the other hand, the first conductive layers 5009 ato 5013 a are hardly etched, and first conductive layers 5017 a to 5021a are formed.

Next, a second doping process is performed. The second conductive layers5017 b to 5020 b are used as masks for an impurity element, and dopingis performed such that the impurity element is added to thesemiconductor layer below the tapered portions of the first conductivelayers. In this embodiment, phosphorus (P) is used as the impurityelement, and plasma doping is performed with a dosage of 1.5×10¹⁴atoms/cm², a current density of 0.5 μA, and an acceleration voltage of90 keV. Thus, low concentration impurity regions 5022 to 5024, whichoverlap with the first conductive layers, are formed in self-aligningmanner. The concentration of phosphorus (P) added to the lowconcentration impurity regions 5022 to 5024 is 1×10¹⁷ to 5×10¹⁸atoms/cm³, and has a gentle concentration gradient in accordance withthe film thickness of the tapered portions of the first conductivelayers. Note that in the semiconductor layers that overlap with thetapered portions of the first conductive layers, the concentration ofthe impurity element slightly falls from the end portions of the taperedportions of the first conductive layers toward the inner portions, butthe concentration keeps almost the same level. Further, an impurityelement is added to the high concentration impurity regions 5014 to5016. (FIG. 6A)

Thereafter, as shown in FIG. 6B a third etching process is performedusing a photolithography method. Mask made of resist 5025 are formed inthe regions where the third etching process is not conducted.Incidentally the tapered portions of the first conductive layers arepartially etched so as to have shapes overlapping the second conductivelayers in the third etching process.

The etching condition in the third etching process is that Cl₂ and SF₆are used as etching gases, the gas flow rate is set to 10/50 sccm, andthe ICP etching method is used as in the first and second etchingprocesses. Note that, in the third etching process, the etching rate forTaN is 111.2 nm/min, and the etching rate for the gate insulating filmis 12.8 nm/min.

In this embodiment, a 500 W RF (13.56 MHz) power is applied to a coilshape electrode under a pressure of 1.3 Pa to generate plasma, therebyperforming etching. A 10 W RF (13.56 MHz) power is also applied to thesubstrate side (sample stage), thereby substantially applying a negativeself-bias voltage. Thus, first conductive layers 5026 a to 5028 a areformed.

Impurity regions (LDD regions) 5029 to 5030, which do not overlap withthe first conductive layers 5026 a to 5028 a, are formed by the thirdetching process. Note that impurity region (GOLD regions) 5022 remainsoverlapping with the first conductive layers 5017 a.

The impurity regions (LDD regions) 5029 and 5030 which do not overlapwith the first conductive layers 5026 a to 5028 a, and the impurityregion (GOLD region) 5022 which overlaps with the first conductive layer5017 a can thus be formed at the same time in embodiment 5, and itbecomes possible to make the regions in response to the properties ofthe TFTs.

The gate insulating film 5005 is etched next after removing a mask 5025formed of resist. CHF₃ is used as an etching gas, and reactive ionetching (RIE) is performed for this etching process. In embodiment 5,the etching process is performed with the chamber pressure set to 6.7Pa, an RF electric power of 800 W, and a CHF₃ gas flow rate set to 35sccm. Portions of the high concentration impurity regions 5014 to 5016are thus exposed, and gate insulating films 5005 a to 5005 d are formed.

A new mask 5031 is then formed from resist, and a third doping processis performed. Impurity regions 5032 and 5033, to which an impurityelement is added, that imparts the second conductivity type (p-type)opposite from that of the first conductivity type (n-type) to thesemiconductor layers, which form active layers of p-channel TFTs; areformed by the third doping process. (See FIG. 3C.) The first conductivelayer 5028 a is used as a mask against the impurity element, impurityelement imparting the p-type conductivity is added, and the impurityregions are formed in a self-aligning manner.

The impurity regions 5032 and 5033 are formed in embodiment 5 by iondoping using diborane (B₂H₆). Note that the semiconductor layers whichform n-channel TFTs are covered by a mask 5031 formed from resist duringthe third doping process. Phosphorous is added to the impurity regions5032 and 5033 in differing concentrations, respectively, by the firstdoping process and by the second doping process. However, doping isperformed such that the concentration of the impurity element whichimparts p-type conductivity to each of the regions becomes from 2×10²⁰to 2×10²¹ atoms/cm³, and therefore no problems will develop with theregions functioning as source regions and drain regions of p-channelTFTs.

Impurity regions are formed in the respective semiconductor layers bythe process up though this point. Note that, although a method forperforming doping of an impurity (B) after etching the gate insulatingfilm is shown in embodiment 5, doping of the impurity may also beperformed without etching the gate insulating film.

The resist mask 5031 is removed next, and a first interlayer insulatingfilm 5034 is formed as shown in FIG. 7A. An insulting film containingsilicon is formed having a thickness of 100 to 200 nm, using plasma CVDor sputtering, as the first interlayer insulating film 5034. A siliconoxynitride film is formed with a film thickness of 150 nm by plasma CVDin embodiment 5. The first interlayer insulating film 5034 is of coursenot limited to a silicon oxynitride film, and other insulating filmscontaining silicon may be used in a single layer or lamination layerstructure.

A process for activating the impurity elements added to each of thesemiconductor layers is performed next. Thermal annealing using anannealing furnace is performed for the activation process. Thermalannealing may be performed in a nitrogen atmosphere having an oxygenconcentration less than or equal to 1 ppm, preferably less than or equalto 0.1 ppm, at 400 to 700° C., typically between 500 and 550° C. Theactivation process is performed in embodiment 5 by heat treatment at550° C. for four hours. Note that, in addition to the thermal annealing,laser annealing and rapid thermal annealing (RTA) can also be applied.

Note also that, in embodiment 5, nickel used as a catalyst duringcrystallization is gettered into the impurity regions 5014, 5015, and5032 containing a high concentration of phosphorous at the same time asthe above activation process is performed. The nickel concentrationwithin the semiconductor layers which mainly become channel formingregions is thus reduced. The value of the off current is reduced forTFTs having channel forming regions thus formed, and a high electricfield effect mobility is obtained because of the good crystallinity.Thus, good properties can be achieved.

Further, the activation process may also be performed before forming thefirst interlayer insulating film 5034. However, when using a wiringmaterial which is weak with respect to heat, it is preferable to performthe activation process after forming the interlayer insulating film 5034(using an insulating material having silicon as its main constituent,silicon nitride film, for example) in order to protect the wirings orthe like, as in embodiment 5.

The doping process may also be performed, and the first interlayerinsulating film 5034 may also be formed, after performing the activationprocess.

In addition, heat treatment is performed for 1 to 12 hours at 300 to550° C. in an atmosphere containing from 3 to 100% hydrogen, performinghydrogenation of the semiconductor layers. Heat treatment is performedfor one hour at 410° C. in a nitrogen atmosphere containingapproximately 3% hydrogen in embodiment 5. This process is one forterminating any dangling bonds of the semiconductor layers by hydrogencontained in the interlayer insulating film 5034. Plasma hydrogenation(using hydrogen excited by a plasma) may be performed as another meansof hydrogenation.

Further, when using a laser annealing method as the activation process,it is preferable to irradiate laser light such as that from an excimerlaser or a YAG laser after performing the above hydrogenation process.

A leveling film 5035 made from a material such as an organic resin isformed next, as shown in FIG. 7B. The leveling film 5035 is formed usingacrylic, which has superior levelness, is used in embodiment 5, and witha film thickness sufficiently capable of leveling steps formed by theTFTs on the substrate. It is preferable that the film thickness be from1 to 5 μm (more preferably, between 2 and 4 μm).

Contact holes are then formed in the first interlayer insulating film5034 and in the leveling film 5035, and wirings 5036 to 5041 are formed.The wirings are formed in embodiment 5 by patterning a lamination filmof a Ti film having a film thickness of 50 nm, and a composite film (acomposite film of Al and Ti) having a film thickness of 500 nm, butother conductive films may also be used. Further, a gate signal line5042 can be formed at the same-time by the same material as the wirings.

A second interlayer insulating film 5043 is formed next by plasma CVDfrom an insulating material containing silicon or from an organic resin.Silicon oxide, silicon nitride, and silicon oxynitride can be used asthe insulating material containing silicon, and materials such aspolyimide, polyamide, acrylic, and BCB (benzocyclobutene) can be used asthe organic resin. Note that it is preferable that the film thickness ofa silicon oxynitride film be from 1 to 5 μm (more preferably, between 2and 4 μm). Silicon oxynitride films are effective in suppressingdegradation of EL elements because the amount of moisture contained inthe film is small.

A contact hole is formed next in order to reach the wiring 5037, and acathode electrode 5044 of a photoelectric conversion element is formed.Aluminum formed by sputtering is used as a metallic film in embodiment5, but other metals, for example, Ti, Ta, W, Cu, and the like can alsobe used. Further, the cathode electrode may also be formed from alamination layer structure composed of a plurality of metallic films,not just a single layer.

An amorphous silicon film containing hydrogen is formed next, and ispatterned, thereby forming a photoelectric layer 5045. A cathodeelectrode 5046 made from a transparent conductive film is similarlyformed over the entire surface, and patterning is then performed to beformed.

Next, as shown in FIG. 8A, a third interlayer insulating film 5047 isformed. A level surface can be obtained by using a resin such aspolyimide, polyamide, polyimide amide, and acrylic as the thirdinterlayer insulating film 5047. In embodiment 5, the polyimide filmhaving a thickness of 0.7 μm was formed.

A pixel electrode 5048 is formed next by forming a transparentconductive film with a film thickness of 80 to 120 nm and thenpatterning, after forming a contact hole in order to reach the wiring5040. (See FIG. 8A.) Note that an indium tin oxide (ITO) film, ortransparent conductive film of indium oxide into which 2 to 20% zincoxide (ZnO) is mixed, is used as the pixel electrode 5048 in embodiment5.

An EL layer 5049 is formed next by evaporation, and in addition, acathode electrode (MgAg electrode) 5050 is formed by evaporation. It ispreferable to perform heat treatment on the pixel electrode 5048 beforeforming the EL layer 5049 and the cathode 5050, thereby completelyremoving all moisture. Note that, although an MgAg electrode is used asthe cathode electrode of the EL element in embodiment 5, other knownmaterials may also be used.

Note also that known materials can be used as the EL layer 5049. A twolayer structure composed of a hole transporting layer and a lightemitting layer is used as the EL layer in embodiment 5, but there arealso cases in which one of a hole injecting layer, an electron injectinglayer, and an electron transporting layer is formed. Many examples ofthese types of combinations have already been reported upon, and any ofthe reported structures may be used.

Polyphenylene vinylene is formed by evaporation as the hole transportinglayer in embodiment 5. Further, a material in which from 30 to 40% of1,3,4-oxydiazole dielectric PBD is distributed in polyvinyl carbazole isformed by evaporation as the light emitting layer, and approximately 1%cumarin 6 is added as a center of green color light emitting.

Further, it is preferable to form a film such as a protective film inorder to protect the EL layer 5049 from oxygen and moisture. A 300 nmthick silicon nitride film is formed as a passivation film 5051 inembodiment 5. The passivation film 5051 may also be formed in successionafter forming the cathode electrode 5050, without exposure to theatmosphere.

Note that the film thickness of the EL layer 5049 may be from 10 to 400nm (typically between 60 and 150 nm), and the film thickness of thecathode electrode 5050 may be from 80 to 200 nm (typically between 100and 150 nm).

An EL module having a structure like that shown in FIG. 8A is thuscompleted. Note that, although the source signal lines are formed by Taand W, the materials which form the gate electrodes, and although thegate signal lines are formed by Al, the wiring material which forms thesource and drain electrodes, in the process of manufacturing an ELdisplay device in embodiment 5 due to the circuit structure and processconsiderations, other materials may also be used.

FIG. 16 is an example of circuit arrangement of a pixel portion in aself light emitting device manufactured in accordance with the processesexplained in embodiment 5. Reference numerals attached to each portionare identical to those used in the equivalent circuit of FIG. 4B. Thelines α-α′, β-β′, and γ-γ′ within FIGS. 5A to 8 correspond to crosssections of portions having the same reference numeral within FIG. 16.

A driver circuit composed of TFTs and the pixel portion shown in FIG. 8Acan be formed on the same substrate by embodiment 5.

Note that light is irradiated below the surface from the EL elements inembodiment 5 (the light irradiation direction is to the TFT substrateside), and therefore a structure is shown in which an n-channel TFT isused for the switching TFT 413 and a p-channel TFT is used for the ELdriver TFT 414. However, embodiment 5 is only one preferred embodiment,and it is not necessary to place any limitations on the structure.

Note also that a structure is shown in embodiment 5 in which the cathodeelectrode 5050 is formed after forming the EL layer 5049 on the pixelelectrode (anode) 5048, but a structure in which an EL layer and ananode are formed on a pixel electrode (cathode) may also be used.Further, at this point it is preferable to structure the switching TFTand the EL driver TFT by n-channel TFTs having low concentrationimpurity regions (LDD regions) as explained in embodiment 5.

However, in this case light is irradiated to the top surface, differingfrom the bottom surface irradiation (in which light emitted from the ELelement is irradiated to the active matrix substrate side on which theTFTs are formed) explained up through this point. One example is shownin FIG. 17. A structure which is the opposite of the structure ofembodiment 5 is thus used in order to conform to the direction light isemitted from the EL element, including a light receiving portion of thephotoelectric element. In addition, the process order is set such thatthe EL layer is formed first, after forming the second interlayerinsulating film 5043, and then the third interlayer insulating film 5047is formed, after which the photoelectric conversion element is formed.

Embodiment 6

Refer to FIG. 13. It is also possible to apply the self light emittingdevice of the present invention having a brightness correction functionto a case in which a display device corresponds to an analog imagesignal. The second image signal (digital image signal) output from thecorrection circuit 1305 is converted to an analog image signal by a D/Aconversion circuit 1314 in this case, is input to the display device1308 corresponding to an analog image signal, and image display isperformed.

A circuit diagram of a source signal line driver circuit in the displaydevice 1308 of FIG. 13 is shown in FIG. 14B. An example of a displaydevice corresponding to an analog image signal is shown here. The sourcesignal line driver circuit has circuits such as a shift register (SR)1411, a level shifter 1412, a buffer 1413, and a sampling switch 1414.Reference numeral 1415 denotes a pixel, reference numeral 1416 denotesthe brightness correction device shown in FIG. 13, and reference numeral1417 denotes a D/A converter circuit.

Operation of each portion is explained. Sampling pulses are output oneafter another from the shift register in accordance with a clock signal(CLK) and a start pulse (SP). The voltage amplitude of the pulse is thenincreased by the level shifter, and the pulse is then output via thebuffer. Correction is performed in the brightness correction device, andthe digital image signal is converted to an analog image signal in theD/A converter circuit, and then input to a video signal line. Thesampling switch is then opened in accordance with the timing of thesampling pulses, the analog image signal input to the video signal lineis sampled, and display of an image is performed by writing voltageinformation into the pixels.

Note that, although the brightness correction device is formed on theoutside of the display device in the example shown in FIG. 13, the twomay also be formed as integrated on the same substrate, as discussed inembodiment 4.

Embodiment 7

In this embodiment, an external light emitting quantum efficiency can beremarkably improved by using an EL material by which phosphorescencefrom a triplet exciton can be employed for emitting a light. As aresult, the power consumption of the EL element can be reduced, thelifetime of the EL element can be elongated and the weight of the ELelement can be lightened.

The following is a report where the external light emitting quantumefficiency is improved by using the triplet exciton (T. Tsutsui, C.Adachi, S. Saito, Photochemical processes in Organized MolecularSystems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437).

The molecular formula of an EL material (coumarin pigment) reported bythe above article is represented as follows.

(M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E.Thompson, S. R. Forrest, Nature 395 (1998) p. 151)

The molecular formula of an EL material (Pt complex) reported by theabove article is represented as follows.

(M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest.Appl. Phys. Lett., 75 (1999) p. 4.)(T. Tsutsui, M. -J.Yang, M. Yahiro, K. Nakamura. T. Watanabe, T. Tsuji,Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn, Appl. Phys., 38 (12B) (1999)L1502)

The molecular formula of an EL material (Ir complex) reported by theabove article is represented as follows.

As described above, if phosphorescence from a triplet exciton can be putto practical use, it can realize the external light emitting quantumefficiency three to four times as high as that in the case of usingfluorescence from a singlet exciton in principle. The structureaccording to this embodiment can be freely implemented in combination ofany structures of the first to sixth embodiments.

Embodiment 8

The EL display which is applied to the light-emitting display device ofthe present invention, is a self-light emitting type, therefore comparedto a liquid crystal display device, it has excellent visible propertiesand is broad in an angle of visibility. Accordingly, the light-emittingdisplay device can be applied to a display portion in various electronicdevices.

The display includes all kinds of displays to be used for displayinginformation, such as a display for a personal computer, a display forreceiving a TV broadcasting program, a display for advertisementdisplay. Moreover, the light-emitting device in accordance with thepresent invention can be used as a display portion of other variouselectric devices.

As other electronic equipments of the present invention there are: avideo camera; a digital camera; a goggle type display (head mounteddisplay); a car navigation system; an acoustic reproduction device (acar audio stereo, an audio component or the like); a notebook typepersonal computer; a game apparatus: a portable information terminal (amobile computer, a portable telephone, a portable game machine, anelectronic book or the like); and an image playback device equipped witha recording medium (specifically, device provided with a display portionwhich plays back images in a recording medium such as a digitalversatile disk player (DVD), and displays the images). In particular,because portable information terminals are often viewed from a diagonaldirection, the wideness of the field of vision is regarded as veryimportant. Specific examples of those electronic equipments are shown inFIGS. 11 to 12.

FIG. 11A shows an EL display containing a casing 3301, a support stand3302, and a display portion 3303. The light emitting device of thepresent invention can be used as the display portion 3303. Such an ELdisplay is a self light emitting type so that a back light is notnecessary. Thus, the display portion can be made thinner than that of aliquid crystal display.

FIG. 11B shows a video camera, and contains a main body 3311, a displayportion 3312, a sound input portion 3313, operation switches 3314, abattery 3315, and an image receiving portion 3316. The light emittingdevice of the present invention can be used as the display portion 3312.

FIG. 11C shows one portion (i.e., a right-hand side) of a head-mountedEL display including a body 3321, a signal cable 3322, a head fixingband 3323, a display unit 3324, an optical system 3325 and a displayportion 3326. The light emitting device of the present invention can beused as the display portion 3326.

FIG. 11D is an image playback device equipped with a recording medium(specifically, a DVD playback device), and contains a main body 3331, arecording medium (such as a DVD) 3332, operation switches 3333, adisplay portion (a) 3334, and a display portion (b) 3335. The displayportion (a) 3334 is mainly used for displaying image information. Thedisplay portion (b) 3335 is mainly used for displaying characterinformation. The light emitting device of the present invention can beused as the display portion (a) 3334 and as the display portion (b)3335. Note that the image playback device equipped with the recordingmedium includes game machines or the like.

FIG. 11E is a goggle type display (head mounted display), and contains amain body 3341, a display portion 3342 and arm portion 3343. The lightemitting device of the present invention can be used as the displayportion 3342.

FIG. 11F is a personal computer, and contains a main body 3351, a casing3352, a display portion 3353, and a keyboard 3354. The light emittingdevice of the present invention can be used as the display portion 3353.

Note that if the luminance of EL material increases in the future, thenit will become possible to use the light emitting device of the presentinvention in a front type or a rear type projector by expanding andprojecting light containing output image information with a lens or thelike.

Further, the above electric devices display often informationtransmitted through an electronic communication circuit such as theInternet and CATV (cable TV), and particularly situations of displayingmoving images is increasing. The response speed of EL materials is sohigh that the above electric devices are good for display of movingimage.

In addition, since the EL display conserves power in the light emittingportion, it is preferable to display information so as to make the lightemitting portion as small as possible. Consequently, when using the ELdisplay in a display portion mainly for character information, such asin a portable information terminal, in particular a portable telephoneor a sound reproduction device, it is preferable to drive the lightemitting device so as to form character information by the lightemitting portions while non-light emitting portions are set asbackground.

FIG. 12A shows a portable telephone, and contains a main body 3401, asound output portion 3402, a sound input portion 3403, a display portion3404, operation switches 3405, and an antenna 3406. The light emittingdevice of the present invention can be used as the display portion 3404.Note that by displaying white color characters in a black colorbackground, the display portion 3404 can suppress the power consumptionof the portable telephone.

FIG. 12B shows an acoustic reproduction device as exemplified by a caraudio stereo, and contains a main body 3411, a display portion 3412, andoperation switches 3413 and 3414. The light emitting device of thepresent invention can be used as the display portion 3412. Further, acar mounting audio stereo is shown in this embodiment, but a portableaudio playback device or a fixed type audio playback device may also beused. Note that, by displaying white color characters in a black colorbackground, the display portion 3414 can suppress the power consumption.This is particularly effective in suppressing the power consumption ofthe portable acoustic reproduction device.

FIG. 12C shows a digital camera, and contains a main body 3501, adisplay portion A 3502, an eye piece portion 3503, and operationswitches 3504, display portion B 3505 and battery 3506. The lightemitting device of the present invention can be used as the displayportion A 3502 and the display portion B 3505.

As described above, the application range of this invention is extremelywide, and it may be used for electric devices in various fields.Further, the electric device of this embodiment may be obtained by usinga light emitting device freely combining the structures of the first toseventh embodiments.

Brightness insufficiencies due to degradation of EL elements, or due toother causes, are corrected on the circuit side by a self light emittingdevice of the present invention, and a self light emitting devicecapable of uniform display on a screen, with no brightnessirregularities, can be provided.

1-31. (canceled)
 32. A light emitting device comprising: means fordetecting brightnesses of light emitting elements of pixels; means forstoring the brightnesses; and means for correcting an image signal basedon the stored brightnesses, wherein an image is displayed using thecorrected image signal, and wherein the correction means performsrelative subtraction processing on image signals written into pixelswhich develop a small reduction in brightness with respect to imagesignals written into pixels which develop a large reduction inbrightness.
 33. The light emitting device according to claim 32, whereinthe storage means comprises a static memory circuit.
 34. The lightemitting device according to claim 32, wherein the storage meanscomprises a dynamic memory circuit.
 35. The light emitting deviceaccording to claim 32, wherein the storage means comprises aferroelectric memory circuit.
 36. The light emitting device according toclaim 32, wherein the storage means comprises a non-volatile memorycapable of being electrically written into, read out from, and erased.37. A light emitting device comprising: means for detecting brightnessesof light emitting elements of pixels; means for storing thebrightnesses; and means for correcting an image signal based on thestored brightnesses, wherein an image is displayed using the correctedimage signal, and wherein the correction means performs relativesubtraction processing on image signals written into pixels which do notdevelop a reduction in brightness with respect to image signals writteninto pixels which develop a large reduction in brightness.
 38. The lightemitting device according to claim 37, wherein the storage meanscomprises a static memory circuit.
 39. The light emitting deviceaccording to claim 37, wherein the storage means comprises a dynamicmemory circuit.
 40. The light emitting device according to claim 37,wherein the storage means comprises a ferroelectric memory circuit. 41.The light emitting device according to claim 37, wherein the storagemeans comprises a non-volatile memory capable of being electricallywritten into, read out from, and erased.
 42. A portable phonecomprising: a main body; a sound input portion; a sound output portion;a display portion; and operation switches, wherein the display portioncomprises a light emitting device, the light emitting device comprises:means for detecting brightnesses of light emitting elements of pixels,means for storing the brightnesses, and means for correcting an imagesignal based on the stored brightnesses, wherein an image is displayedusing the corrected image signal, and wherein the correction meansperforms relative subtraction processing on image signals written intopixels which develop a small reduction in brightness with respect toimage signals written into pixels which develop a large reduction inbrightness.
 43. The portable phone according to claim 42, wherein thestorage means comprises a static memory circuit.
 44. The portable phoneaccording to claim 42, wherein the storage means comprises a dynamicmemory circuit.
 45. The portable phone according to claim 42, whereinthe storage means comprises a ferroelectric memory circuit.
 46. Theportable phone according to claim 42, wherein the storage meanscomprises a non-volatile memory capable of being electrically writteninto, read out from, and erased.
 47. A portable phone comprising: a mainbody; a sound input portion; a sound output portion; a display portion;and operation switches, wherein the display portion comprises a lightemitting device, the light emitting device comprises: means fordetecting brightnesses of light emitting elements of pixels, means forstoring the brightnesses, and means for correcting an image signal basedon the stored brightnesses, wherein an image is displayed using thecorrected image signal, and wherein the correction means performsrelative subtraction processing on image signals written into pixelswhich do not develop a reduction in brightness with respect to imagesignals written into pixels which develop a large reduction inbrightness.
 48. The portable phone according to claim 47, wherein thestorage means comprises a static memory circuit.
 49. The portable phoneaccording to claim 47, wherein the storage means comprises a dynamicmemory circuit.
 50. The portable phone according to claim 47, whereinthe storage means comprises a ferroelectric memory circuit.
 51. Theportable phone according to claim 47, wherein the storage meanscomprises a non-volatile memory capable of being electrically writteninto, read out from, and erased.